Closing Huge threads affect on Server load?

Hi
we have a vb board with about 1.3 million posts and we have many problem with shared hosting plan and high server load.
I saw in some boards they close huge threads and start new part 2 of this thread. but old thread still in the forum and viewable but is not open for posting.
I want to know this changes affects on server load or not? If yes we have some active threads with 10 000 pages !
second question is: If some cache softwares installed on server reduce server load? and whats the best cache script?

We did this some time ago when threads were reaching 500 posts. I didn't really notice any gain by doing this, and was later told it didn't matter. Not sure if anyone has done any real testing on this in a controlled environment such as a test board with a fixed number of people online.

We have boards with threads as big as 2000 replies and tested a few times. No gain, but there is a loss, that people cant access the old thread and if your threads are spidered by Search Engines, you loose some traffic there.

My advice, get a better server, one just for you (dedicated), and you will be in full control off any software installed there. Plus, you will be able to really optimize it for your needs.